An in-mold expansion-molded article obtained by filling expanded polypropylene resin particles into a mold and thereafter heating the mold with water vapor to mold the resin particles, is characteristic in its selectability of shape, its lightweight properties, its heat-insulating properties and like properties, each of which are merits of the in-mold expansion-molded article. Moreover, upon comparison with in-mold expansion-molded articles made of similar expanded synthetic resin particles, the in-mold expansion-molded article made of the expanded polypropylene resin particles shows better chemical resistance, heat resistance, and distortion recovery rate after the article is compressed, as compared to an in-mold expansion-molded article made of expanded polystyrene resin particles. Furthermore, the in-mold expansion-molded article made of the expanded polypropylene resin particles shows better dimensional accuracy, heat resistance, and compression strength as compared to an in-mold expansion-molded article made of expanded polyethylene resin particles. From these characteristics, the in-mold expansion-molded article made of the expanded polypropylene resin particles are used for various uses such as heat-insulating material, cushion packaging material, interior members for automobiles, and core material for a car bumper.
On the other hand, in order to mutually fuse the expanded polypropylene resin particles together in the mold to obtain the in-mold expansion-molded article, there is the need to heat the particles at a higher temperature than that for heating the expanded polystyrene resin particles or for heating the expanded polyethylene resin particles. That is to say, there is the need of a high molding heating vapor pressure. This causes the need of a mold and a molding machine which can tolerate high pressure, and also causes a problem of high vapor cost required for the molding.
Most molding machines for in-mold expansion molding of expanded polypropylene resin particles have a pressure resistance upper limit of approximately 0.4 MPa. The expanded propylene resin particles used for the in-mold expansion molding is resin which have properties corresponding to this limit; typically, a propylene random copolymer having a melting point of 140° C. to 150° C. is used. However, due to the substantial increase of fuel prices these days, there are demands for further reduction of the molding heating vapor pressure.
One available method of lowering the molding heating vapor pressure is a method of using a propylene random copolymer resin including more comonomer content and having a further lower melting point (e.g. a method of using a propylene random copolymer having a melting point of not more than 140° C.). However, the melting point of a propylene random copolymer generally has a positive correlation to resin rigidity; if a resin having a low melting point is used, the rigidity of the resin decreases. This causes physical properties of the resin such as compression strength to decrease when the resin is made into an in-mold expansion-molded article, and further shrinking and deformation of the in-mold expansion-molded article tends to occur more remarkably.
On the contrary, if a propylene random copolymer resin including less comonomer content and having a high melting point is used to achieve a high rigidity, the melting point of the resin increases, which causes the molding heating vapor pressure required for obtaining a good in-mold expansion-molded article to increase. Hence, if a higher rigidity is required, there is the need to use a highly pressure-resistant molding machine and mold. This causes an increase in equipment costs and further an increase in molding processing costs due to the increase in utility usage mainly of water vapor and the like.
In order to solve these problems, proposals have been made to use a propylene/1-butene random copolymer or a propylene/ethylene/1-butene random terpolymer (Patent Literature 1, Patent Literature 2), or a metallocene polypropylene (Patent Literature 3) as resin having a low melting point while having a relatively high resin rigidity as compared to the melting point. However, the propylene random copolymer including 1-butene comonomer is slow in the polymerization speed of 1-butene. This causes poor polymerization productivity, and also has a problem that the resin cost is high. Moreover, with the metallocene polypropylene, its catalytic promoter is expensive, while the resin cost is also high. Furthermore, melt fractures easily generate when preparing mini pellets by an extruding granulation technique. This causes a problem of having low extrusion productivity.